3-Acetoxymethyl-7.beta.-aminoceph-3-em-4-carboxylic acid (7-ACA) is of primary importance as a starting material for the preparation of semi-synthetic commercial cephalosporin antibiotics. This intermediate is produced commercially from cephalosporin C, a fermentation product of Cephalosporin acremonium (see Newton and Abraham, Nature, 175, 548, 1955), by hydrolysis of the D-.alpha.-aminoadipyl side chain, either chemically or enzymaticially. 3-Hydroxymethyl-7.beta.-aminoceph-3-em-4-carboxylic acid (des-7-ACA) is usually produced as an unwanted side product in the preparation of 7-ACA caused by either the chemical or enzymic (esterase) hydrolysis of the 3-acetyl group. Des-7-ACA can be produced by the action of various esterases on 7-ACA. In addition, it can be produced from desacetyl-cephalosporin C by enzymic hydrolysis in a manner analogous to the conversion of cephalosporin C to 7-ACA (World Patent No. WO 90/12110). As a result of the highly reactive nature of the C-7 amino group, chemical synthesis of 7-ACA from the deacetylated precursor results in a mixture of products. The use of typical chemical acylating reagents such as acetyl chloride or acetic anhydride produces inter alia 7-ACA, N-acetyl 7-ACA and N-acetyl desacetyl 7-ACA. In addition, 3-hydroxymethyl-7.beta.-aminoceph-3-em-4-carboxylic acids are susceptible to lactonization under these conditions. Prior art methods of O-acylation involve blocking the reactive C-7 amino group, then performing O-acylation in a non-polar, organic solvent in the presence of a 4-(tertiary amino) pyridine catalyst with an acid-acceptor base and then deblocking the C-7 amino group (European Patent No. 153,874A). Further methods comprise of esterifying the 4-position carboxyl group of the C-7 acylated compound to prevent lactonization during the reaction, and deesterifying the carboxy group and deprotecting the C-7 amino group to produce the 3-alkanoyloxymethyl-7.beta.-aminoceph-3-em-4-carboxylic acid (U.S. Pat. No. 3,532,694), or performing the O-acylation in an aqueous medium in the presence of a 4-(tertiary amino)pyridine catalyst with an acid-acceptor base and then deblocking the C-7 amino group (European Patent No. 0230972). More recently, a chemical process for acetylating the 3-hydroxymethyl cephalosporins in an aqueous solvent has been described (U.S. Pat. No. 5,221,739), as well as the use of an enamine for the protection of the C-7 amino group (U.S. Pat. No. 5,552,542). Thus, it was unexpectedly discovered by the present inventors that the use of certain acetyl donors in the presence of an activator or catalyst can provide an efficient direct chemical acetylation of desacetyl-cephalosporin without the use of any protecting or blocking groups.